1. Field of the Invention
This invention relates to data processing systems and more particularly to data processing operations requiring the transfer of information over a common input/output bus or via a communication channel wherein information is transferred from one data processing system to another.
2. Description of the Prior Art
Various methods and apparatus are known in the prior art for transferring information from one computer system to another computer system or from one device in a given computer system to another device in the same computer system over a common input/output bus. Most prior art devices in transferring such information require the action of the central processing unit as an intermediary and communicate to other units via the central processing unit (CPU). More advanced means for communication within a system or between systems provide for direct communication from one unit to the other unit without the intervention of the central processing unit (CPU). One such system is disclosed in U.S. patent application Ser. No. 591,964, filed on June 30, 1975 and entitled, "Apparatus for Processing Data Transfer Requests in a Data Processing System" which is assigned to the same assignee as the instant application and which issued into U.S. Pat. No. 3,993,981 on Nov. 23, 1976 and is hereby incorporated by reference to the instant application. That system provides for a plurality of devices coupled over a common bus whereby bidirectional transfer of information may be provided between such devices coupled to the bus such as one or more data processors, one or more memory units, various types of peripheral devices, such as magnetic tape storage devices, disk storage devices, card reading equipment and the like. Additionally, a data processing system utilizing a common bus is shown in U.S. Pat. No. 3,815,099.
Communication of computer systems with each other is described in a book entitled "Communication Networks for Computers", by D. W. Davies and D. L. A. Barbour published in 1973 by John Wiley and Sons of New York, N.Y. One problem when computer systems communicate with each other or when devices within a computer system communicate with each other surfaces when words of different lengths or different formats are utilized by each system or device. For example, in the instant invention, bidirectional transfer of information from an NML controller is attached to an HNP bus whereby the NML controller handles sixteen bit words and the HNP bus handles eighteen bit words. Furthermore, in many instances, the HNP memory requires that words stored therein be right-justified within sixteen contiguous bits comprised of 2 eight-bit words and an A bit (the first bit from the left) and a B bit (the ninth bit from the left) filling in the high order end of the word. Although, in this instance, the two words are eighteen bit and sixteen bit lengths, the words may typically be of any other length and would give rise to the same problem.
Another problem which presents itself in the bidirectional transfer of information arises when a source unit requests a read operation be performed in any one of a number of memories at an address provided by the source unit. Since there are many devices attached to a common bus system, main memory must have some means for identifying the source unit in order to return the information to the proper device.